1. Field of the Invention
The present invention relates to a perimeter, and more particularly to a perimeter for projecting a stimulus on the inner surface of a visual field dome and for recording the response of a subject relating to a projected position of the stimulus and the visual recognition of the stimulus.
2. Description of the Prior Art
Known in the art is a perimeter for measuring a visual field, in which an illuminated spot is projected as a stimulus on a visual field dome that has a hemispherical projection surface. The projected position of the stimulus is successively varied by an examiner or automatically by the apparatus, and the perimeter records a response of the subject relating to the visual recognition of the stimulus (for example, see Japanese Laid-open Patent Application No. 2002-272685).
In such a perimeter, the position of the stimulus is controlled by a motor, and the stimulus is displayed at a substantially uniform interval, although a certain range of interval variation will be experienced in the examination. Therefore, when the stimulus is displayed, motor sound or other forms of operation sound generated by the perimeter will also be present at a substantially uniform interval. The examiner will accordingly check to see whether the subject is reliably responding by being able to see the stimulus, or is merely responding to the motor sound, and thereby assesses the reliability of the examination results.
For this purposes, with the stimulus not being displayed, only the motor used to display the stimulus is driven at a certain set frequency (e.g., once for every 50 times the stimulus is displayed). If the subject responds in such circumstances, it is assumed that the subject is responding to the motor sound, not because they have seen the stimulus.
A ratio is obtained for the number of times the subject responded when only motor sound was generated. A higher ratio indicates a lower level of examination result reliability. Such a reliability test is referred to as a false-positive test, and is a method traditionally used with perimeters for performing static visual field examinations.
In addition to the false-positive test, another method is used with perimeters for performing static visual field examinations. In this method, the reliability of the examination results is determined by deliberately projecting a stimulus that will definitely be visible to the subject according to the nature of their response during the examination. This reliability test is referred to as a false-negative test. For example, assuming that the sensitivity is measured to be 20 dB at certain viewing conditions (e.g., color and size) at a certain projection position during the examination, a stimulus that is more readily visible than this stimulus is projected at 0 dB (maximum brightness) as a false-negative stimulus at the indicated position. A response is ordinarily expected from the subject in this case. However, in cases where the subject does not respond, it is assumed that they have, e.g., lost the ability to focus, become fatigued, or started to produce inaccurate responses. The following ratio is then provided: (number of non-responses for false-negative stimulus display)/number of times of false-negative stimulus display). A higher ratio indicates lower reliability for an examination result.
The false-positive and false-negative tests described above are static tests wherein the projected stimulus does not move. However, in the visual field examinations performed using a perimeter in, e.g., Japanese Laid-open Patent Application No. 2002-253502, which discloses a kinetic visual field examination, a stimulus projected in a visual field dome is moved in order to provide a kinetic stimulus, and a visual field examination is performed on the basis of the time the subject takes to respond to the moving stimulus. In such examinations as well, it is preferred for a false-positive test or false-negative test to be performed, and the reliability of the kinetic examination results to be evaluated.
It is therefore an object of the present invention to provide a perimeter that is capable of performing a reliable false-positive or false-negative test in a kinetic visual field examination and capable of evaluating the reliability of the kinetic examination results.